The layer of mucus on the surface of bluegills (Lepomis macrochirus) captured in Lake Biwa was characterized as 1) large enough to host microbes (ca. 76 mm thick), 2) a physically different environment from the surrounding lake water in viscosity and buffering capacity, and 3) chemically rich in organic substances, which may be utilized as nutrients. Based on DAPI staining and on the number of colonies formed respectively, it was found that ca. 10 3 times and 3 to 7 times the number of microbial cells were present in the mucus layer, as compared with the lake water. The bacterial flora of the mucus was greatly different from that of the lake water, according to a phylogenetic analysis. About 60% of the isolates from the mucus were Gram-positive. These Gram-positive isolates could be divided into two major groups. Each group consisted of strains sampled in one season, i.e., the strains sampled in July were closely related to the genus Staphylococcus, while the strains sampled in November were close to the genus Mycobacterium. In contrast, most isolates from the lake water were Gram-negative (72%); with all the strains closely related to band g-Proteobacteria sampled in July. With the exception of one strain, the Gram-positive isolates from the lake water (6 strains) were all sampled in November. Almost all of the isolates from the mucus could metabolize glucose, whereas only half of the isolates from the lake water could do the same.

The mucus obtained from the surface of bluegills (Lepomis macrochirus) can be utilized as a source of nutrients by the bacterial strains isolated from the mucus and the adjacent lake water. The isolates from both the mucus and the lake water grew better in 20-fold diluted mucus than in lake water. Almost all mucus isolates (10 of 12 strains) hardly grew at all in lake water, whereas half (6 of 12 strains) of the lake-water isolates were able to grow in lake water, according to the increase in optical density (O.D.) after culturing in mucus or lake water. The isolates grew faster in 20-fold diluted mucus than in lake water, with a greater enhancement of the growth rate for the mucus isolates than the lake-water isolates. The features of the mucus isolates, i.e., little growth in lake water and faster growth in mucus, were deduced to be responsible for the different bacterial flora of the fish mucus. The performance of 20-fold diluted mucus as a source of nutrients was evaluated by comparing it with nutrient broth (NB) medium and 100-fold diluted NB medium (DNB medium). In many strains, the amount of growth (maximum O.D.) was highest in NB, then mucus and lastly DNB, coinciding with the order in terms of the concentrations of glucose equivalent, i.e., NB, 20-fold diluted mucus and DNB. About 60% and 70% of the isolates grew faster in the 20-fold diluted mucus than in the NB and DNB mediums, respectively. The growth yield (increase in O.D. when the unit amount of glucose equivalent was consumed) was on average, ca. 70 times greater in the 20-fold diluted mucus than NB. The growth-rate constant in the mucus did not differ markedly among the isolates belonging to various phylogenetic groups. By contrast, in the NB medium, the isolates closely related to the Actinobacteria and a-Proteobacteria had smaller growth-rate constants and the isolates closely related to gProteobacteria had greater growth-rate constants. The respiratory activity of the isolates was greatly enhanced by adding a small amount of mucus (1% as the final concentration), due to unknown factor(s). Thus, as a source of nutrients, the mucus seemed to be more effective, as well as being able to sustain the growth of various bacterial groups.